A current mirror is a basic building block in the design of a direct current (DC) bias network, which may be used in a radio frequency (RF) power amplifier (PA). In some applications, e.g., a radio in a wireless local area network (WLAN), a PA is required to be pulsed on and off during operation at a relatively high frequency. This is accomplished by pulsing bias networks within the PA on and off.
As a bias network within the PA is pulsed on and off, the output conductance of a transistor within a current mirror of the bias network may vary as the voltage across the bias network ramps up and down. This results in a time-dependent bias network output voltage, which, in turn, results in a time-dependent bias current for a main transistor of the bias network. As the bias current varies over the bias network pulse, the RF gain of the PA's pulse may also vary. This results in time-dependent amplitude modulation (AM)—AM distortion and AM—pulse modulation (PM) distortion.
In order to simplify the communication systems, most demodulators only track amplitude at the beginning of a PA's pulse. Therefore, any change in the RF gain over the PA's pulse will degrade an error vector magnitude (EVM), which is used to measure a performance of a radio transceiver.